The invention relates to a transmission method used in a radio system comprising a transmitter and a receiver, which method comprises transmitting signals by means of which a connection is set up between the transmitter and the receiver and which propagate along different paths to the receiver, and in which method the signals that have propagated along different paths are combined in the receiver after the signals have been received.
Cellular radio systems generally employ different diversity methods to improve the quality of a connection between a base station and a subscriber terminal under varying conditions. The base station can transmit a signal to the subscriber terminal in the downlink direction by means of several different antennas. Diversity combining decreases particularly the disadvantages caused by signal fading. In diversity methods, information contained in preferably several received signals that do not correlate with one another is combined. In multipath reception, the most common diversity receivers combine the signals before or after the detection. Multipath signals are usually detected for example by Viterbi detection.
Connection quality between a base station and a subscriber terminal usually depends on, for example, the speed at which the subscriber terminal moves and the distance between the transmitting antennas. Another factor affecting the connection quality is for example obstacles on the radio path that reflect the signal, thus causing a delay therein.
The receiver arrangement generally used in radio systems is a so-called RAKE receiver that consists of one or several RAKE branches. Each branch is an independent receiver unit the function of which is to compose and demodulate one received signal component. Each branch can be synchronized with a signal component that has propagated along an individual path. In a conventional receiver, the signals from the receiver branches are combined to obtain a signal with good quality. However, combining of signals does not always provide the best possible signal. In some cases, the signal components forming the sum signal are received at a stage where summing of the signals deteriorates the signal capacity.
In known radio systems, downlink antenna beams of a base station are directed on the basis of a signal a subscriber terminal has transmitted to the base station. Such a downlink beam steering method is called a DOA (Direction of Arrival) method. The beam steering is carried out on the basis of a signal transmitted by a subscriber terminal to a base station, even though the uplink and downlink signals do not have the same frequency, which means that the steering is not accurate. Since the downlink and the uplink direction are not identical in practice, phase calibration is required between TX and RX parts of a base station. If several antennas are used simultaneously for signal transmission, the summed antenna beam of the antennas will have a very strip-like shape, which causes problems especially in signal reception. An antenna beam that is common to several transmitting antennas also contains shadow areas, which cause problems on a connection between the base station and a subscriber terminal.
The object of the present invention is to provide a transmission method and a radio system with which the aforementioned problems can be solved.
This is obtained with a transmission method of the type described in the introduction, characterized in that at least two signals are transmitted for each connection and the signals are combined in the receiver into a signal that forms the connection and that is measured, and in which method the optimality of the phase of the transmitted signal is estimated and the phase of at least the signal transmitted in the downlink direction is adjusted on the basis of the measurement and estimation.
The invention also relates to a radio system comprising a transmitter and a receiver that communicate with each other by means of signals they have transmitted, which signals propagate along different paths from the transmitter to the receiver, which comprises means for combining the signals that have propagated along different paths after the reception.
The radio system according to the invention is characterized in that the radio system comprises transmitting means for transmitting for each connection at least two signals that are combined in the means into one signal, measuring means for measuring the combined signal and for estimating the optimality of the transmission, and control means that adjust, on the basis of the measurement and estimation, the phase of at least the signal that is transmitted in the downlink direction during the signal transmission by the transmitting means in order to decrease the accumulation of signals during the combining of the signals.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that the antenna beam of the transmitter is changed on the basis of data received from measurement of the signals received by the receiver, so that the receiver can receive an optimal signal.
The transmission method and the radio system according to the invention provide several advantages. By means of the method, signals received by the receiver are adjusted on the basis of measurement reports provided by the receiver. The adjustment can be carried out rather slowly, if necessary. The method can improve the performance in the downlink direction by about 3 to 8 dB, depending on the situation. The adjustment is also executed adaptively, such that the performance in the downlink direction does not decrease below the performance of a non-combined signal, which is possible in conventional methods. Since the antenna beam of the transmitter is directed on the basis of a signal received by the receiver, the receiver receives an optimal signal. By means of the method, even a mobile subscriber terminal can receive a signal of good quality that is adapted to the subscriber terminal.